The hardware support for additional serial ports is definitely there. The hardware design of the controller board was modeled after the Arduino Mega, and we chose to bring pins out to the prototyping headers based upon their capabilities on the Mega. In looking at the pinout of the headers (Sheet 2 of the Controller Board schematic), IIRC the pins D2-D5 and D14-15,D18-19 were brought out specifically because they had the capability of being used as serial ports.
So you’re going to have to write some custom code that utilizes some of these pins as a serial port, and include it in the Controller Arduino code. The tougher bit is then passing that data to the BeagleBone and then to the browser. This has been discussed before; if you do a search there should be some threads about how to port an Arduino program to the OpenROV. Our concept from the beginning was that one should be able to do development on an Arduino Mega, then port it to OpenROV by replacing calls to the terminal session with calls to the OROV telemetry stack.